To study dosimetric effects in time-dependent geometries the results of individual three-dimensional (3D) calculations are usually added or statistically combined. This is cumbersome if the geometry is complex, if high time resolution is envisaged (e.g. considering dose rate), or if the microscopic volume displacement as a function of time is not known. Furthermore, it is difficult to study double-dynamic systems to investigate the influence of time-dependent beam delivery (moving leafs in a multi-leaf collimator) on the dose deposition in a moving target (organ motion or deformation). Here, 4D instead of multiple 3D calculations are required. Monte Carlo (MC) simulations are believed to be the most accurate tool for dose calculation and wiII presumably have the biggest impact in areas of large density variations, e.g. for lung cancer treatments. Interestingly, this is also the area where we might expect the biggest impact of organ motion on the dose distribution. The MC method is well suited to study dosimetric effects of motion with high accuracy. However, until now, like for analytical dose calculation techniques, the geometric information cannot be modified during the simulation. This limits applications to multiple 3D instead of true 4D. In taking advantage of objejctoriented C++ programming techniques we are proposing true 4D MC dose calculation. Local dose deposition in the patient will be calculated while beam configuration and organ geometry are changed continuously MLC leaf positions will be changing during the simulation according to leaf sequencing files. Based on deformable image registration we will track microscopic areas based on the patient's CT during the dose calculation. 4D dose calculation will allow the usage of a preset moving pattern based on clinical experience or patient specific 4D CT information. The capability of studying time dependent geometries and the interplay in double-dynamic systems for any time scale based on treatment head variations and patient's breathing pattern will take MC dose calculation to a new level, to the level of 4D treatment simulation.